Clinical evidence suggests that cellular immunity plays a role in controlling HIV-1 replication Using the simian immunodeficiency virus of macaques (SIVmac) animal model of AIDS, we show that virus replication was not controlled in monkeys depleted of CD8+ lymphocytes during primary SIVmac infection Eliminating CD8+ lymphocytes form monkeys during chronic SIVmac infection resulted in a and dramatic increase in viremia that was again suppressed coincident with the reappearance of SIVmac-specific CD8+ T cells These results confirm the importance of cell-mediated immunity in controlling HIV-1 infection and support vaccination approaches that will elicit these immune responses for preventing infection